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1991 Mk2 GTI 8v Engine stalls when fuel is below 1/4 after riding for a while

I have a problem with my 1991 Mk2 GTI 8v. After riding for a while with my GTI when I'm low of fuel (less than 1/4, but without getting to the reserve) there's a point when the engine stalls and I can't turn it on until 10-15 minutes. Usually before the engine starts to show symptoms of stalling I can hear a very noisy sound that looks to come from one of the fuel pumps.

Also when the engine gets hot at normal temperature and I want to accelerate fast the throttle does not response, it does nothing for some seconds (but 10 or more seconds, sometimes it works one second yes, one not, one yes... for a time) until it "works" again.

I cleaned the ISV, replaced spark plugs, distributor cap and distributor rotor which where in bad condition, but it didn't fix the problem.

I don't know how cars were equipped in Spain but in the US they came with a fuel pump in the fuel tank and one in a black box just in front of the rear axle on the passenger side. The in-tank pump runs around $50 US new. It is attached to the fuel sending unit in the gas tank. It is a simple job to replace it using common tools but you do have to work in gasoline. The other fuel pump, located in the black fuel reservoir under the car, goes bad less frequently and is much more expensive (Around $200 US). That pump is also not bad to replace but once again involves working with gasoline.

Your symptom of working OK until approx 1/4 tank is indicative of a bad in-tank pump. The in-tank pumps go bad often. That is where I would start. One of the in-tank pump failure modes is also intermittant running -- running until getting hot and then shutting down. A failed in-tank pump can also damage the main pump since the main pump operates in a reservoir of gasoline which keeps it cool. If your in-tank pump has failed you may be stressing the main pump and it will soon also fail. I have seen cars with bad in-tank pumps run for a long time as long as the gas tank is kept opver 1/4 tank. Apparently the main pumps can get sufficient fuel for the engine and cooling in that situation.

You could check the fuel pressure -- there is a test barb at the end of the fuel rail. But if the in-tank pump is working on and off or not at all the main pump might not have enough fuel supply to produce full pressure. There is also a flow test that can be done with the in-tank pump to see if it is pumping enough gas. I usually just flow the pump output into a spare gas can instead of trying to measure quantity.

The fuel system has a timed shut off after a few seconds if the engine is not running so the fuel pump relay is pulled and the two large perpindicular slots under it jumpered for test purposes. FR

I don't know how cars were equipped in Spain but in the US they came with a fuel pump in the fuel tank and one in a black box just in front of the rear axle on the passenger side. The in-tank pump runs around $50 US new. It is attached to the fuel sending unit in the gas tank. It is a simple job to replace it using common tools but you do have to work in gasoline. The other fuel pump, located in the black fuel reservoir under the car, goes bad less frequently and is much more expensive (Around $200 US). That pump is also not bad to replace but once again involves working with gasoline.

Your symptom of working OK until approx 1/4 tank is indicative of a bad in-tank pump. The in-tank pumps go bad often. That is where I would start. One of the in-tank pump failure modes is also intermittant running -- running until getting hot and then shutting down. A failed in-tank pump can also damage the main pump since the main pump operates in a reservoir of gasoline which keeps it cool. If your in-tank pump has failed you may be stressing the main pump and it will soon also fail. I have seen cars with bad in-tank pumps run for a long time as long as the gas tank is kept opver 1/4 tank. Apparently the main pumps can get sufficient fuel for the engine and cooling in that situation.

You could check the fuel pressure -- there is a test barb at the end of the fuel rail. But if the in-tank pump is working on and off or not at all the main pump might not have enough fuel supply to produce full pressure. There is also a flow test that can be done with the in-tank pump to see if it is pumping enough gas. I usually just flow the pump output into a spare gas can instead of trying to measure quantity.

The fuel system has a timed shut off after a few seconds if the engine is not running so the fuel pump relay is pulled and the two large perpindicular slots under it jumpered for test purposes. FR

First thank you for your answer.

Actually I don't know if my golf has 1 or 2 fuel pumps. I'll confirm the number of pumps later to be sure.

Looking in a car parts webpage I can see some pumps with a great difference of price, some cost around 70-90€ an others arround 250€ so I can suposse Spanish Golfs have 2 pumps...

For now I'll try with the fuel filter replacement with the hope that it will fix the problem .

Right. But Digifant cars didn't have the high pressure pumps required of CIS cars. The in-tank pump is simply a transfer pump from the gas tank to the main pump fuel reservoir.

Test flow the in-tank pump and see what you get. The pump is accessed through a round access panel in the rear hatch floor. I usually simply disconnect the fuel line, connect a fuel line of an appropriate size and then aim the line into a gas can or other container. Connect the pump to a 12V DC power supply and watch it flow. In 10 seconds the pump should deliver 300 cc of fuel (10 oz). I rarely measure the flow. If the flow is strong the pump is ok -- if it dribbles or has a weak flow replace it. One other test I usually perform is to jumper the fuel pump relay and test for 12V DC at the fuel pump. FR

Having more gas in the tank makes the life of the in-tank pump easier for sure and may provide enough of a hydraulic head to supply the main pump without the in-tank pump even operating.

Since both pumps depend on being immersed in gasoline for cooling -- your in-tank pump may well be complaining full time and overheating when the gas level is below 1/4 tank. Lack of sufficient fuel over time will also harm the main pump and that pump will have to be replaced.

As a general "rule" I never let my gas tank get too far below 1/4 tank to protect the fuel pump(s).
After the initial investment, it costs no more to maintain a gas tank at half full than to keep it closer to empty. FR

I replaced the fuel filter and I have done some test on the car. Looks like it functions better, but it still stalls when it wants and it's still being a but "unstable". This time I've gone around the city with heavy traffic (the worst) and worked for one hour with heavy traffic, then it stalls for 5 minutes and then it engaged (15 seconds of start engine :/ ) and worked for 20 more minutes until I get home.

Another thing I discovered, but I'm not sure, is that the blue coolant sensor (ECS) is faulty or something because when I unplug it nothing happens and as I know when you disconnect it the RPM increase. This has been done with the engine in cold temperature, and the engine idle was at 950 rpm (a bit unstable with bad sound).

Can be possible that blue sensor is the source of some problems? I checked the resistance of the sensor in cold (25ºC) and it was 2010 Ohms.

The 2100 Ohms sounds about right for the 25C temp. More important is that the temp sensor resistance varies with the temperature of the engine. With the engine hot the resistance should be 200-300 Ohms. Those blue sensors cost less than $20 US. They are very easy to replace -- make sure you use a new "O" ring gasket and be sure to remove the old gasket from the hole. Leave the cooling system sealed and remove the old one and replace it with the new one quickly. Do it with the engine overnight cold. A small amount of coolant will spill so be sure to have a catch pan for it.
Top up the coolant if need be.

I continue to think you have fuel pump issues especially based on the FP noises you reported. Running on and off is a fuel pump failure mode. They run for a while and then shut down when they get hot then will work again after they cool off. However, that symptom could also be indicative of a bad fuel pump relay or even a Digifant Relay. The relays have a similar failure mode. (Run for awhile -- get hot -- shut down -- cool off -- run for a while.) FR

The 2100 Ohms sounds about right for the 25C temp. More important is that the temp sensor resistance varies with the temperature of the engine. With the engine hot the resistance should be 200-300 Ohms. Those blue sensors cost less than $20 US. They are very easy to replace -- make sure you use a new "O" ring gasket and be sure to remove the old gasket from the hole. Leave the cooling system sealed and remove the old one and replace it with the new one quickly. Do it with the engine overnight cold. A small amount of coolant will spill so be sure to have a catch pan for it.
Top up the coolant if need be.

I continue to think you have fuel pump issues especially based on the FP noises you reported. Running on and off is a fuel pump failure mode. They run for a while and then shut down when they get hot then will work again after they cool off. However, that symptom could also be indicative of a bad fuel pump relay or even a Digifant Relay. The relays have a similar failure mode. (Run for awhile -- get hot -- shut down -- cool off -- run for a while.) FR

If the main pump is going noisy before stalling, the fuel pump relay should be fine.

CTS is of no concern at the moment. If of interest, check to see that you are in closed loop when warm (voltmeter to o2 sensor to check for oscillation). If it drives sluggishly until it's warmed up, CTS may be of interest.

Just do the in-tank fuel pump. As Ziddey states (and as I have stated several times) the fuel pump noise you report is a good indicator that is where your issues are.

To answer your question: The fuel pump relay is on the fuse and relay panel -- lower right corner -- first relay row above the fuses. Look for a PN like 191 906 383. Here in the US they can be purchased on line for well under $10. I always used to carry a spare. FR

The fuel pick-up strainer looks pretty good. You might want to use a flashlight and take a look inside the gas tank to see how much crud is in there. If there is a lot of stuff floating around in the tank you might want to remove the tank from the car and clean it out.

Mount the new pump in place of the old and reassemble. Use new hose clamps in place of any you have had to remove. Make sure that the pump fits well on the assembly so it doesn't leak. FR

Today I tested the fuel pump and simply doesn't work, no sound or movement when I put +12v to it (red wire = + and brown = - and test vice versa too). I checked continuity of everything and all plugs or internal wires are OK. When you put +12v to the pump it should have started to work, even on a dry environment, right?

Can the 3.5 Bar fuel pump deliver fuel when the in-tank pump is completly broken?

Tomorrow I'll get a pressure meter so I can check the fuel presure. I'll mount everything and check the presure of the fuel ramp.

Hey but if the tank fuel pump was dead how was able the other pump to get fuel? The plastic box near the fuel filter is an small reservoir from where the 3.5Bar pump gets the fuel, and that small reservoir is filled thanks to the in-tank pump, right?